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I've found these be most useful to me. Others may have differing opinions or other recommendations.
The Estuary Flyfisher by Steve Raymond, 1996, Frank Amato Publications
Perhaps the single-most useful book for anyone flyfishing the Sound. Excellent work. A definite "Must Have."
Fly Fishing For Pacific Salmon by Bruce Ferguson, Les Johnson & Pat Trotter, 1985, Frank Amato Publications
Excellent. The chapters on Salmon Life Histories and Salmon Feed are absolutely outstanding and extremely valuable for learning about and understanding our Pacific Salmon. Excellent Fly Types and Patterns chapter. When I bumped into Bruce Ferguson at Doc's several months ago, he said an updated version was soon to be published. Hard to imagine how this work can be improved upon.
Saltwater Fly Fishing for Pacific Salmon by, Barry Thornton, 1995, Hancock House
Contains excellent information on locating salmon which will be most useful to the boating flyfisher in the Sound. Although his focus is on British Columbia, the tactics and techniques he presents are equally applicable to the Sound. He also has several excellent articles and info online at
http://www.bcadventure.com/adventure/angling/protalk/thornton/thornton.phtml.
Salmon to a Fly: Fishing for Pacific Salmon in the Open Ocean by Jim Crawford, 1995, Frank Amato Publications
Focus is on British Columbia but generally good info if you can't do enough reading about flyfishing for Pacific Salmon.
Fly Fishing Afoot in the Surf Zone by Ken Hanley, 2nd publishing 1999, Frank Amato Publications. Originally written in 1994.
Short, sweet and right to the point. Doesn't get into flyfishing the Sound at all, but has good, useful info should you take your fly rod with you on your next trip to the coast.
Saltwater Flies: Over 700 of the Best by Deke Meyer, 1995, Frank Amato Publications
Though not strictly dedicated to the PNW waters, its still an excellent collection of the most productive saltwater patterns with recipes. Great for getting new ideas.
Tube Flies - A Tying, Fishing & Historical Guide by Mark Mandell & Les Johnson, 1995, Frank Amato Publications
Great book for learning just about everything you ever wanted to know about tube flies. Many saltwater flyfishers on every coast are turning to them because of their effectiveness and increased number of solid hookups. Silvers don't take flies head on; tube flies move the hook farther back behind the fly. Enough said?
Greg